Conventional ac-dc converters for high power applications such as dc transmission use thyristor valves for ac-dc conversion They can operate as rectifier or inverter by choosing appropriate values for the firing angles. Since the direction of current flow through these valves is in only one direction, the polarity of the voltage across the dc terminals, established by adjustment of the firing angles, determines the direction of the power flow. This is true not only for the three phase bridge circuit used for dc transmission, but also for any type of ac-dc converters using thyristor valves.
A conventional 6-pulse three phase bridge circuit is shown in FIG. 1(a). It is symbolically represented by FIG. 1(b). In principle, FIG. 1(b) could represent any ac-dc converters using thyristor valves, for example, the 12-pulse converter typically used for dc transmission or the single phase converters used for low voltage dc power supplies.
FIG. 2 is a schematic diagram of a two terminal dc transmission system. In such a circuit, the power direction can be easily changed by changing the polarity of the voltage of the dc bus by adjusting the firing angles of the converters of the two terminals. However, in a multi-terminal dc system, the problem of changing the power directions becomes more complicated. Take the example of a three terminal system shown in FIG. 3. The converters A and B are operating as rectifiers and converter C is operating as an inverter. If one wants to change the direction of power in one or two of the three converters without changing the direction of power in the remaining one(s), it is not possible to do so by changing the polarity of the dc bus alone. In the circuit shown in FIG. 3, if the power direction in converter A alone is to be changed, it can be done by polarity reversal switches as shown in FIG. 4. If all converters are to have the flexibility to change the direction of power without necessarily changing the direction of power in the other connected converters, similar arrangement for polarity reversal switching must be provided at all converters. Such polarity reversal arrangement requires additional switches and increased insulation requirements for the converter transformers, bus work and/or valve structures. The necessary switching operations introduce delays in effecting power reversals.
In a bipolar dc transmission system, one method of enhancing the availability of the system is to provide for paralleling of the converters on the two poles to operate in monopolar mode when there is an outage of one pole of the transmission line or cable. Such situations would also require polarity reversal switching arrangements for the converters.
The present invention eliminates the need for polarity reversal switches and higher insulation requirements for converter equipment in such situations. It also provides additional benefits as will be explained later.